1. Field of the Invention
The present invention relates generally to a cathode ray tube and more particularly to a cathode ray tube with an electron beam reflection film in the form of a bismuth oxide thin film having a high bulk density on the electron beam collision side of a color selective electrode assembly and a method of producing the same.
2. Description of the Prior Art
When three electron beams emitted from an electron gun in a typical cathode ray tube are projected on a phosphor layer formed on the inner face of a panel portion through electron beam passing openings of a color selective electrode assembly such as a shadow mask a pixel portion of the phosphor layer onto which the electron beams are projected becomes luminous, so that the phosphor layer as a whole displays a desired colored image.
In such an image display, the transmittance of the electron beams through the aforementioned shadow mask is of the order of from 10 to 20%. However, the electron beams which have failed to pass through the electron beam apertures of the color selective electrode assembly and collided with the color selective electrode assembly flow in the color selective electrode assembly in the form of an electric current, thus causing the color selective electrode, assembly to undergo thermal expansion because of the Joule s heat resulting from the current. As a result, the positional relationship between the color selective electrode assembly and the phosphor layer formed on the inner face of the panel portion slightly varies and the electron beams projected onto the phosphor layer commit landing errors. The landing efforts of the electron beams cause a shift in color in the display image and this phenomenon is called mask doming. Due to the mask doming thus caused in the display image on the cathode ray tube, not only the purity of color of the display image but also the white uniformity thereof may greatly deteriorate.
A known mask-doming suppressing means used in such a cathode ray tube is adapted to reducing the electron beam energy given to the color selective electrode assembly, that is, suppressing the thermal expansion of the color selective electrode assembly by employing a metal having a low thermal expansion coefficient such as invar for the color selective electrode assembly and coating the electron beam collision side of the color selective electrode assembly with an electron beam reflection film, whereby the quantity of mask doming is lowered.
There are a first, a second, a third and a fourth method of forming the aforementioned electron beam reflection film as disclosed in Japanese Patent Laid-Open Nos. 80438/1988, 80439/1988, 75132/1990 and 283526/1987, respectively.
According to the first method above, bismuth oxide (Bi2O3) put on an evaporation cell of stainless steel is subjected to radio-frequency heating, and deposited by vacuum deposition on a shadow mask.
According to the second method above, bismuth oxide (Bi2O3) on a tungsten boat is subjected to resistor heating, and deposited by vacuum deposition on a shadow mask.
According to the third method above, a sintered pellet of bismuth (Bi) powder on a tungsten boat is subjected to resistor heating and bismuth (Bi) is deposited by vacuum deposition on one side of a shadow mask as an electron beam reflection film.
According to the fourth method above, suspension of bismuth oxide (Bi2O3) powder together with slurry containing water glass acting as a binder is sprayed by means of a spray-gun so as to form a coating layer of bismuth oxide (Bi2O3) on one side of a shadow mask as an electron beam reflection film.
According to the first method, a vacuum deposition apparatus is generally complicated because a substrate is a color selective electrode assembly made of electric conductor and the mass-productivity is therefore low. According to the first method, moreover, part of bismuth oxide (Bi2O3) chemically reacts with the stainless steel of the evaporation cell since the evaporation cell of stainless steel is heated up to about 900xc2x0 C. and the reaction product is simultaneously deposited on the color selective electrode assembly likewise. In an extreme case, further, the bismuth oxide (Bi2O3) chemically reacts with the stainless steel of the evaporation cell and is reduced, causing a drawback that the metal bismuth (Bi) thus reduced is also deposited on the color selective electrode assembly. As the melting point of the metal bismuth (Bi) thus deposited is low (about 270xc2x0 C.), small balls of bismuth (Bi) (so-called bismuth balls) are formed on the color selective electrode assembly during the heat treatment (about 400 to 450xc2x0 C.) of the process of manufacturing a cathode ray tube. Therefore, there is another problem, arising from the deterioration of the electric insulating property of such a cathode ray tube, that bismuth balls are separated by vibration and the like.
According to the second method, the heating temperature has to be higher than that in the first method since the resistor heating of the tungsten boat is used to heat the bismuth oxide (Bi2O3). The tungsten boat thus heated up chemically reacts with the bismuth oxide (Bi2O3) and raises the melting temperature, whereby substances lower in density than bismuth oxide are produced. Moreover, a highly porous bismuth oxide layer is formed because the bismuth oxide evaporated in a low vacuum region at a pressure of 10xe2x88x922 Torr attracts and absorbs the residual gas such as oxygen or nitrogen and water vapor. Since impurities are thus deposited, the film structure is nonuniform, making it difficult to form a dense film and impossible to form a uniform thin film particularly when the film thickness is of the order of micrometers or lower. Consequently the electron reflection effect is greatly deteriorated. Since the tungsten boat is used to heat the bismuth oxide (Bi2O3) as in the case of the first method, part of the bismuth oxide (Bi2O3) chemically reacts with the tungsten of the evaporation boat, so that the reaction product is also deposited on the color selective electrode assembly. As in the case of the first method, further, the bismuth oxide (Bi2O3) chemically reacts with the tungsten of the evaporation boat and is reduced in an extreme case and the bismuth (Bi) thus reduced is also deposited on the color selective electrode assembly, whereby bismuth balls are formed on the color selective electrode assembly. Therefore, there has been the same problem, as what arises in the first method, that the electric insulation property of the cathode ray tube deteriorates and the bismuth balls are separated by vibrations and the like.
According to the third method of forming the electron beam reflection the melting point of the bismuth (Bi) is normally about 270xc2x0 C., which is lower than the temperature of the heat treatment during the process of a manufacturing cathode ray tube. Therefore, the bismuth (Bi) film formed and deposited on one side of the color selective electrode assembly melts during the manufacturing process and due to the surface tension, the bismuth becomes spherical and is turned to bismuth balls. When the bismuth balls adhere to the electron beam passing openings of the color selective electrode assembly, the electron beam passing openings are stopped therewith, and mask aperture holding occurs in the color selective electrode assembly. The third method which is liable to cause mask aperture blocking of the color selective electrode assembly brings about pixel blemish fatal to a fine pitch color cathode ray tube that requires high density and high resolution image display. Moreover, the use of expensive sintered pellets of bismuth (Bi) produces a problem of increased cost when such an electron beam reflection film is formed.
According to the fourth method, further, the suspension of bismuth oxide (Bi2O3) is employed as a material to be sprayed when the electron beam reflection layer is formed, which makes coarser the particles of the bismuth oxide (Bi2O3) coating layer that has been formed and thicker the coating layer, whereby the shapes of the electron beam passing openings formed in the color selective electrode assembly become uneven. If the shapes of the electron beam passing openings become uneven, halation increases and the fidelity of the mask pattern lowers, whereby the purity of color and white uniformity of the display image deteriorate. The fourth method that brings about such a deterioration in characteristics still has a problem leading to performance deterioration fatal to a fine pitch color cathode ray tube that requires high density and high resolution image display.
A first object of the present invention is to provide a cathode ray tube comprising a color selective electrode assembly provided with an electron beam reflection film, and capable of high density and high resolution image display.
A second object of the present invention is to provide a method of producing a cathode ray tube comprising a color selective electrode assembly provided with an electron beam reflection film, and capable of high density and high resolution image display.
A cathode ray tube according to the present invention comprises a panel portion with a phosphor layer formed on its inner face, an electron gun for projecting electron beams toward the phosphor layer, a neck portion accommodating the electron gun, a funnel portion coupling the panel portion to the neck portion, and a color selective electrode assembly which has electron beam passing openings arranged opposite to the phosphor layer with a space therebetween, and is provided within the panel portion. An electron beam reflection film of a bismuth oxide (Bi2O3) thin film having a bulk density of at least 1 g/cm3, e.g., in a range of from1 to 9.3 g/cm3, more specifically, from 4 to 9.3 g/cm3, is formed on the face of the color selective electrode assembly against which the electron beam collide. In the cathode ray tube according to the present invention, the bismuth oxide thin film of the electron beam reflection film is from 5 to 700 nm thick
A method of producing a cathode ray tube according to the present invention comprises the steps of placing a high-density-pressed pellet of bismuth oxide (Bi2O3) powder or bismuth oxide (Bi2O3) powder on a boat of a vacuum deposition apparatus comprising a vacuum chamber, a boat whose sample stage side is made of platinum or a platinum alloy containing at least one of iridium, osmium, palladium, rhodium and ruthenium, a color selective electrode setting stage, heating means for heating the boat, and evacuation means, mounting the color selective electrode assembly on the color selective electrode setting stage, evacuating the vacuum chamber down to 10xe2x88x924 Torr, vaporizing the bismuth oxide (Bi2O3) pellet or the bismuth oxide (Bi2O3) powder by use of the heating means, and depositing a bismuth oxide (Bi2O3) thin film on one side of the color selective electrode assembly as an electron beam reflection film having a bulk density of at least 1 g/cm3, e.g., from 1 to 9.3 g/cm3, more specifically, from 4 to 9.3 g/cm3. The method of producing the cathode ray tube according to the present invention includes the step of depositing a bismuth oxide (Bi2O3) thin film on the side of the color selective electrode assembly as an electron beam reflection film having a bulk density of at least 1 g/cm3, e.g., from 1 to 9.3 g/cm3, more specifically from 4 to 9.3 g/cm3, using a vacuum deposition apparatus equipped with a sample stage of which the sample stage side is a boat made of platinum or a platinum alloy and having a generally trapezoidal shape.